Simplify this bit of code!
Re-work how transmit buffer limits are enforced - partly to fix the PR,
but partly to just tidy up things.

The problem here - there are too many TX buffers in the queue! By the
time one needs to transmit an EAPOL frame (for this PR, it's the response
to the group rekey notification from the AP) there are no ath_buf entries
free and the EAPOL frame doesn't go out.

Now, the problem!

* Enforcing the TX buffer limitation _before_ we dequeue the frame?
  Bad idea. Because..
* .. it means I can't check whether the mbuf has M_EAPOL set.

The solution(s):

* De-queue the frame first
* Don't bother doing the TX buffer minimum free check until after
  we know whether it's an EAPOL frame or not.
* If it's an EAPOL frame, allocate the buffer from the mgmt pool
  rather than the default pool.

Whilst I'm here:

* Add a tweak to limit how many buffers a single node can acquire.
* Don't enforce that for EAPOL frames.
* .. set that to default to 1/4 of the available buffers, or 32,
  whichever is more sane.

This doesn't fix issues due to a sleeping node or a very poor performing
node; but this doesn't make it worse.

Tested:

* AR5416 STA, TX'ing 100+ mbit UDP to an AP, but only 50mbit being received
  (thus the TX queue fills up.)
* .. with CCMP / WPA2 encryption configured
* .. and the group rekey time set to 10 seconds, just to elicit the
  behaviour very quickly.

PR:		kern/138379
Add a sysctl vfs.read_min to complement the exiting vfs.read_max.  It
defaults to 1, meaning that it's off.

When read-ahead is enabled on a file, the vfs cluster code deliberately
breaks a read into 2 I/O transactions; one to satisfy the actual read,
and one to perform read-ahead.  This makes sense in low-latency
circumstances, but often produces unbalanced i/o transactions that
penalize disks.  By setting vfs.read_min, we can tell the algorithm to
fetch a larger transaction that what we asked for, achieving the same
effect as the read-ahead but without the doubled, unbalanced transaction
and the slightly lower latency.  This significantly helps our workloads
with video streaming.

Submitted by:	emax
Reviewed by:	kib
Obtained from:	Netflix